Takeshi Yamao

Refractive Index along the Molecular Long Axis of an Orthorhombic Thiophene/Phenylene Co-oligomer Crystal

We have determined a refractive index along the crystal long axis of an orthorhombic thiophene/phenylene co-oligomer (TPCO) crystal. From the space group symmetry of the crystal, the three crystallographic axes are identical to the principal axes of the refractive indices. We estimated the index to be 3.76 on the basis of the mode intervals of the equally-spaced longitudinal multimode laser oscillation which is highly polarized along the crystal long axis (i.e., the a-axis). This index is much larger than those associated with the other axes. These experimental indices were compared to the semiempirically calculated ones, in qualitatively good agreement with them.

Improved Field-Effect Transistor Characteristics of an n-Type Semiconducting Thiophene/Phenylene Co-Oligomer

We studied the organic field-effect transistor (OFET) characteristics of n-type thiophene/phenylene co-oligomers chosen as an organic semiconductor layer. The n-type characteristics were achieved by introducing trifluoromethyl groups on either molecular terminal. With crystals of this material, we fabricated OFET devices for which an MgAg alloy was used for drain and source electrodes and polymer layers were used as a gate insulator. In particular, the device fabricated on a poly(vinyl phenol) gate insulator layer shows a high electron mobility of 1.5 cm2 V-1 s-1. That on a poly(methyl methacrylate) layer indicates a low threshold voltage of 4.1 V. The polymer gate insulators and the low work-function metal play an important role in achieving a good device performance.

Single-Crystal Growth and Charge Transport Properties of an Alternating Co-Oligomer Composed of Thiophene and Phenylene Rings

We grow single crystals of a thiophene/phenylene co-oligomer (TPCO) composed of seven alternating thiophene and phenylene rings (AC7). These crystals are grown in a liquid phase directly onto silicon substrates covered with a silicon dioxide layer. The grown crystals are well-defined hexagons and are composed of a molecular layered structure along the molecular length. Top-contact field-effect transistors are fabricated by depositing metal electrodes on the AC7 crystals grown on the substrates. We estimate the field effect mobilities in the transistor device configurations. The mobility data are associated with the crystallographic results of the material. The devices produce a maximum mobility of ~0.1 cm2V-1s-1. This value is higher than that obtained for another TPCO composed of five alternating rings (AC5). The present results indicate that AC7 crystals are potentially useful for electrical device applications.

Organic Light-Emitting Field-Effect Transistor with Channel Waveguide Structure

Organic field-effect transistors with channel waveguide structures were fabricated with thiophene/phenylene co-oligomer thin films. Top-contact source/drain electrodes of gold were deposited onto both side walls of the waveguide by the self-shadow masking vapor deposition technique leaving the co-oligomer channel of ~5-µm-length intact. The device operated as a typical hole-accumulated unipolar transistor under negative gate biases. When alternating-current gate voltages were applied, the device showed light emission due to recombination of injected electrons with accumulated holes in the waveguide channel.

Melt Molding of Uniaxially-Oriented Crystal Domains onto a Substrate

We have fabricated semiconducting oligomer films composed of uniaxially-oriented crystal domains by melt molding. It is highly important to generate a temperature gradient in the lateral direction parallel to the substrate on which the said oligomer films are fabricated. As a result, the melted oligomer layer starts to solidify from parts of lower temperatures to end up as a uniaxially aligned film. The uniaxial alignment is further promoted by slowly cooling that oligomer layer. In the present studies, the temperature gradient was set at ~0.06–0.4 °C/mm. From the polarizing microscope observations, we confirm that within this range the temperature gradient efficiently produces the uniaxial orientation. This method has been applied to fabricating organic field-effect transistors. Those devices show an effective modulation of the drain current with varying gate voltage.